An SCR catalyst is used as an example to investigate the effects of the washcoat diffusion limitation on the performance of monolith reactors. One-dimensional model and three-dimensional CFD model with and without washcoat diffusion limitation were established. The washcoat diffusion was modeled by using an effective diffusivity in the washcoat region (CFD modeling) and the internal mass transfer coefficient between the interface and the interior of the washcoat (One-dimensional modeling). The results show that numerical models with washcoat diffusion limitation give more accurate NO concentration prediction than the numerical models without washcoat diffusion limitation. Using the ratios of internal mass transfer resistance and reaction resistance to estimate the washcoat diffusion limitation, the correlation between the temperature and the washcoat limitation is discussed. Detailed comparisons of the 1-D model and CFD model are conducted.